The manufacture of internally plated seamless steel tubes has up to now been generally performed in the manner that a hollow block formed of a support material and a plating material is shaped by extrusion so as to form a tube. Initially, a cylindrical block of a support material such as low-alloy steel is drilled in an axial direction so as to create a hollow block. A cylindrical block of a plating material such as high alloy steel and of the same length and of a diameter corresponding to the inside diameter of the hollow block formed of the support material is also drilled in the axial direction and inserted into the hollow block of said support material.
The two hollow blocks are placed one within the other and are welded together at their ends so that the annular slot between the two hollow blocks is tightly closed off so that the contact surfaces of the hollow blocks do not oxidize upon heating to the extrusion temperature and prevent a suitable bond between the support material and the plating material.
However, this procedure has serious disadvantages. The weld on the ends represents a weak point which can tear, for instance, upon heating so that the contact surfaces may nevertheless oxidize. In addition, the preparation of a plated hollow block which is ready for use requires considerable expense on the one hand, due to the working (i.e., drilling, welding) necessary and, on the other hand, the considerable use of expensive plating material (i.e., waste produced upon the drilling).
For the manufacture of a steel plate which is plated on one side, the applicant has already proposed a method (Federal Republic of Germany P 39 07 903) in which the plating material is applied in molten state to a support plate. For this purpose, the flat sides of two support plates are placed tightly on one another and dipped into a melt of the plating material until a sufficiently thick layer of plating has formed by crystallization.
However, this procedure of applying a layer of plating directly from the molten state onto the support material cannot be directly applied to the manufacture of plated hollow blocks. Upon the immersion of a hollow block of the support material into a melt of the plating material, a plating layer would immediately form on the inner surface as well as the outer surface. This outer layer is not necessary and would greatly increase the cost of manufacture due to the unnecessary consumption of plating material. In order to avoid outside plating, it is possible to fill a hollow block of support material with a melt of the plating material or, in order to keep the consumption of plating material as small as possible, centrifuge it, for instance, with a layer of this material and allow it to solidify. In such a case, however, the problem arises that due to different thermal expansion and shrinkage, the plating layer detaches itself from the support material before the further processing of the plated hollow block can take place.